In designing a key guide, some provision must be incorporated to restrict side-to-side movement of the key. This movement results from side force. Side force is a vector component of the overall force applied to a key. This overall force may be applied by an external source, e.g., the finger of the operator, and/or by an internal source, e.g., a return spring. Gravity also contributes to side force--particularly if the key axis of motion is not vertical or if the key is unbalanced. The side force vector component is perpendicular to the key axis of motion. Besides cancelling the side force vector, three features of operation are generally desirable in a key guide:
1. Silent operation: Silent operation is an unachieved feature when the key is mounted as in FIG. 1. The key stem designated by reference numeral 1 is inserted into a frame 2 formed of rigid yet low friction material such as nylon. The holes 3, 4 in the frame must be slightly larger than the key stem to minimize friction. Because of this clearance, or "leeway room", side force often will result in slight sideways movement of the stem. When the stem contacts the inside of one of the holes, this movement is decelerated and halted. The force exerted by the stem against the inside of the hole during this deceleration period sends a shock wave through the frame.
Since the frame is formed of substantially rigid material, the deceleration period is very brief and the resultant shock wave carries a high frequency component of relatively large amplitude. The large high frequency component is perceived audibly as a "click" sound. Since the frame may act as a sounding board, even a very slight clearance may allow sufficient wobble to produce an audible rattle during operation. Such noise is particularly undesirable when the keyboard is employed for a musical application, such as an electronic synthesizer, organ, etc.
2. Low friction: Friction increases when the noise problem is addressed as in FIG. 2. This drawing figure shows a side-view cross section of a bushing modified to reduce noise. Here the key stem 1 slides through a grommet 5 which is inserted in a hole in the frame 2. The grommet is formed of a soft material such as silicone rubber. Since the grommet absorbs much of the force of lateral impart, this design affords quieter operation that the guide shown in FIG. 1. However, since the key stem is sliding against a softer material, friction is increased. This increased friction is particularly a problem when the key is one of a row of keys in a musical keyboard. Musicians sometimes wish to play glissandos, a technique which imparts significant lateral as well as vertical force on the keys. When this lateral force results in high friction, glissandos may be difficult or impossible to perform.
A felt ring may be substituted for the rubber grommet, but this would decrease friction only slightly, if at all. Furthermore, felt introduces a new set of problems: Felt tends to wear and compact with time, and the labor cost of affixing felt would likely surpass that of inserting a grommet.
3. Stable horizontal key position: Stable horizontal key position is an unachieved feature when the friction problem is solved as in FIG. 3. Here the key stem 1 slides through a bushing 6 which is inserted in a grommet 7. The bushing is formed of a low friction material such as nylon. The grommet is formed of a soft material such as silicone rubber.
If the grommet is soft enough, and if the sound absorbtion width 8 is sufficiently wide, then rattle caused by lateral key stem motion will be minimal. However, these conditions will increase the lateral distance which the key stem may move as a result of side force. Consequently, the keyboard feels sluggish.
If the grommet 7 is engineered so that the sound absorbtion width 8 is small, allowing little lateral key movement, then the grommet will not significantly attenuate the high frequency shock wave created when the key stem 1 contacts the bushing 6.
With the aforementioned art, one cannot design a key guide which features, at once, durability, silent operation, stable horizontal key position and low friction.